Electrical traction combined with traditional thermal traction for making hybrid traction is becoming increasingly more popular in road vehicles.
In order to integrate an electrical machine in a road vehicle, an adequate cooling system dedicated to the electrical machine should be provided to prevent the electrical machine from overheating, and in particular to prevent the insulation of the conductors forming the stator winding from exceeding the maximum allowable temperature (the even partial melting of the insulation of the conductors forming the stator winding has a destructive effect because it causes irreversible short-circuits). It is a further object of the cooling system of the electrical machine to maintain the working temperature of the electrical machine relatively low in order to increase the efficiency of the electrical machine itself (the specific electrical resistance of the copper usually used in the stator windings increases with temperature, thus keeping lower the temperature of the electrical machine reduces the electrical resistance of the stator windings and thus losses in the stator windings due to Joule effect).
The most common cooling systems use air or water, but the use of mineral oil is becoming more frequent (typically by exploiting the same lubrication and cooling oil as the transmission oil). The use of mineral oil instead of water has some advantages because the mineral oil can also ensure the lubrication of mechanical components (mainly bearings) of the electrical machine without needing to interpose sealing devices, but also some drawbacks because mineral oil has a lower heat removal capacity than water (normally this lower heat removal capacity is compensated by keeping the temperature of the mineral oil lower, with a consequent oversizing of the cooling circuit).
Patent application DE102008049226A1, which is incorporated by reference, describes a cooling system of an electrical machine of a vehicle; the cooling system comprises an outer casing, which encloses the casing of the electrical machine therein, defining an annular cooling chamber about the electrical machine, through which a cooling fluid is made to circulate. The cooling system described in patent application DE102008049226A1 has some disadvantages, because it is not simple to be assembled about the casing of the electrical machine and does not have particularly high thermal performance (i.e. with a quantity of heat disposed per unit of circuited fluid being relatively low).
Patent application GB2217118A, which is incorporated by reference, describes a cooling system of an electrical machine having a plurality of helicoidal cooling channels which are obtained in a casing containing the electrical machine; in particular, the casing consists of an inner shell and an outer shell which are inserted into each other and in which the helicoidal cooling channels are obtained. The cooling system described in patent application GB2217118A has some drawbacks because it does not allow to cool the stator winding headers in optimal manner.